Dental adhesives such as bonding materials and cements are divided, based on the curing mechanism, into dental adhesives of photopolymerization type which are polymerized and cured by irradiating them with visible rays and dental adhesives of chemical polymerization type which are polymerized and cured by mixing two or more compositions separately stored, immediately before using.
The performance of the dental adhesives of photopolymerization type bonded to tooth substances has been rapidly improved in recent years. The reason is considered to be that polymerization inhibition by oxygen in air in the polymerization process is relatively small by virtue of this curing mechanism and even under the acidic conditions the rate of polymerization is enhanced. On the other hand, in order to cure the dental adhesives of photopolymerization type, presence of light is an essential requisite, so that in the case where an adhesive is applied to the place which light hardly reaches, it is necessary to use a dental adhesive of chemical polymerization type.
A large number of proposals relating to the dental adhesives of chemical polymerization type have been made so far, but as compared with the performance of the dental adhesives of photopolymerization type, performance of the dental adhesives of chemical polymerization type cannot be said to be satisfactory. The reason is considered to be that the polymerization initiation efficiency of a polymerization initiator used is low under the acidic conditions, and in the polymerization process, the dental adhesives of chemical polymerization type are subject to polymerization inhibition by oxygen, so that the rate of polymerization is not sufficiently enhanced.
For such dental adhesives of chemical polymerization type, there has been made a proposal of using a polymerization initiator which is easily polymerized and cured even under the acidic conditions (patent document 1 (Japanese Patent Laid-Open Publication No. 97306/1995)). Further, in order that a dental adhesive may hardly suffer polymerization inhibition by oxygen in the polymerization process, there has been made a proposal of increasing viscosity of a dental adhesive or using a paste of high viscosity by mixing a dental adhesive with a filler or the like (patent document 2 (Japanese Patent Laid-Open Publication No. 291819/1995)).
In the patent document 1, an adhesive curable composition which is cured through chemical polymerization by mixing a liquid composition with a powder composition immediately before using has been proposed, but it has been indicated that when N-phenylglycine (NPG) used as one component of a polymerization initiator is mixed with a polymerizable monomer, gelation takes place in a short period of time.
In the patent document 2, there has been proposed an adhesive curable composition which is used in the following manner. That is to say, N-phenylglycine (NPG) or sodium p-toluenesulfinate (p-TSNa) used as one component of a polymerization initiator is adsorbed on a jig or the like and stored, while the composition containing a polymerizable monomer and a filler is stored in the form of a paste, and the paste in a given amount and the jig having adsorbed the polymerization monomer, etc. are brought into contact with each other immediately before using. In this proposal, it is necessary to prepare jigs with different adsorbed amounts according to the amounts of the adhesive curable composition used, and in the case of a jig with a fixed adsorbed amount, there is a problem that the amount of the adhesive curable composition used is markedly restricted. Together with this proposal, a method of storing a mixture of NPG and a polymerizable monomer having an oxyethylene chain in a molecule has been proposed.
However, if such a composition is stored for a long period of time, the composition is discolored, and a color tone of a cured product is sometimes impaired.
It is an object of the present invention to provide a reaction-curable adhesive composition containing a composition capable of stably storing a polymerizable monomer over a long period of time, said polymerizable monomer generally undergoing gelation reaction in a short period of time, suffering occurrence of extreme discoloration and being considered to have bad storage stability.
It is another object of the present invention to provide a dental adhesive kit using such a stabilized reaction-curable adhesive composition as above and having excellent long-term storage stability.